What Hermeticism says about the pattern in things
Hermeticism is largely a claim about pattern: that the same structure repeats at every scale, so what you can see tells you something about what you cannot. Two things are worth knowing first. The older texts here are the Emerald Tablet and the Corpus Hermeticum, while the famous set of seven principles comes from The Kybalion, a book published in 1908 rather than in antiquity. The library cites each accordingly, and the lines below follow it.
The Principle of Correspondence
As above, so below. The claim is that the planes of existence mirror one another, so a law you can learn on the level you observe counts as evidence about levels you cannot.
The Emerald Tablet
The One Thing
Everything is said to come from one source by adaptation, which makes the variety of the world variation rather than difference in kind.
The Emerald Tablet
The Microcosm and Macrocosm
If correspondence holds, a person is a small model of the cosmos and self-knowledge becomes a way of studying it. The library attributes the phrase about man as a little world to Democritus, so the idea is older than the Hermetic texts.
Fragment
The Three Great Planes
Reality is set out on three planes, physical, mental and spiritual, each with endless gradations and all linked by correspondence.
The Kybalion, Chapter VIII
The Principle of Vibration
Nothing is at rest. Matter, energy and mind are treated as the same thing at different rates of motion, which is what licenses talking about one in terms of another.
The Kybalion, Chapter IX
The Principle of Polarity
Opposites are described as one scale with two ends. Hot and cold, love and hate, differing by degree rather than in kind.
The Kybalion, Chapter X
The Principle of Rhythm
Everything swings out and back. The practical use offered is timing: work with a tide rather than against it.
The Kybalion, Chapter XI
The Principle of Cause and Effect
Nothing is put down to chance. Every effect is held to have its cause, and the chain is said to run on indefinitely in both directions.
The Kybalion, Chapter XII
Cosmic Sympathy
Things that correspond are said to be bound to each other, so acting on one can reach the other. This is where the scheme stops being description and becomes the basis for magical practice.
Hermetic Tradition
The Music of the Spheres
The planets are described as producing a harmony, meaning proportion rather than audible sound. The library traces this to the Pythagorean tradition rather than to the Hermetic texts.
Pythagorean Tradition
The appeal of this scheme is also its weakness: a pattern that fits everything cannot be caught out by anything. Read as physics it does not survive contact with the evidence. Read as a way of moving between what you know and what you are trying to understand, it is doing something else, and the tradition's own instruction is to test it in practice rather than take it on trust.